Model Based Control of Refrigeration System



Traditional control of refrigeration system in the supermarket is to control the air temperature inside refrigerated display cabinets to be within a specific hysteresis band. Activation and deactivation of refrigeration is typically based on the air temperature measured in the airflow of the cabinet. This is due to a lot of reasons, historically or technically, the focus of refrigeration has been on maintaining a desired air temperature, not an optimal product temperature. One of the main reasons can be the complexity of product temperature measurement, and its direct application to controller. While the strict requirement from food authority regarding safety, ongoing demand from consumers on product quality as well as its transparency, high expectations from supermarket regarding simple set up and energy saving , product quality improving, all push the manufacturer of refrigeration systems to construct some new control solutions. One of the possible solutions could be to integrate the food product models into the model of a refrigeration system to find the optimal temperature profile. Foodstuff is stored not only for long life but also fresh. The ideal situation also includes simplifying the set up procedure and making the scientific data visible to the consumer and authorities, and at the same time, keeps the energy consumption at a low level. Food deterioration and temperature. Due to the nature of the food as a physicochemically and biologically active system, food quality is a dynamic state continually moving to reduced levels. When temperature is lowered, the deterioration rate in general is reduced. Food deterioration is an accumulated result of temperature variation over time, not only a single temperature at one point of time, for some short period of time, such as the defrost cycle, exposing to a higher temperature out of the range is not fatal, but the defrost profile such as the defrost frequency, its duration and temperature level will definitely affect its storage quality. Frost and defrost. Frost formation is a well known undesirable phenomenon in the refrigeration system. Frost affects the performance of the heat exchanger by decreasing the effective air flow area and increasing the thermal resistance between the warmer air and the cold refrigerant inside the evaporator. This decrease in free flow area results in a lower air flow and an increase in the pressure drop across the coil. This performance degradation will become severe with time if nothing is done, in the worst situation the system will be iced up and break down. In order to maintain a satisfactory performance, the evaporator in the display cabinet needs to be defrosted regularly. While the traditionally applied electric heater for frost removal by melting will introduce the following side effects: increase the energy consumption and reduces the accuracy of the temperature control for the refrigerated display cabinet, so to increase the rate of food deterioration. Summing up, on one side, we need to find an optimal temperature profile for the food storage with optimal defrost control scheme; another side, we need also to consider the overall energy consumption. One solution is to build these multi objectives into cost function, and construct the new control scheme to fulfil the overall optimization requirement. Expected results: Temperature quality A control scheme that optimise the food quality with respect to energy consumption Easy set up and adjustment of the refrigeration system control Ability to produce transparent scientific data to consumer and authorities Overall optimal energy consumption
Effektiv start/slut dato01/10/200430/09/2007


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